Re: [RFC PATCH 0/2] KVM: Support Heterogeneous RT VCPU Configurations.

[Peter Zijlstra]

On Wed, Jul 28, 2021 at 04:36:58PM +0900, Suleiman Souhlal wrote:
> Hello,
> 
> This series attempts to solve some issues that arise from
> having some VCPUs be real-time while others aren't.
> 
> We are trying to play media inside a VM on a desktop environment
> (Chromebooks), which requires us to have some tasks in the guest
> be serviced at real-time priority on the host so that the media
> can be played smoothly.
> 
> To achieve this, we give a VCPU real-time priority on the host
> and use isolcpus= to ensure that only designated tasks are allowed
> to run on the RT VCPU.

WTH do you need isolcpus for that? What's wrong with cpusets?

> In order to avoid priority inversions (for example when the RT
> VCPU preempts a non-RT that's holding a lock that it wants to
> acquire), we dedicate a host core to the RT vcpu: Only the RT
> VCPU is allowed to run on that CPU, while all the other non-RT
> cores run on all the other host CPUs.
> 
> This approach works on machines that have a large enough number
> of CPUs where it's possible to dedicate a whole CPU for this,
> but we also have machines that only have 2 CPUs and doing this
> on those is too costly.
> 
> This patch series makes it possible to have a RT VCPU without
> having to dedicate a whole host core for it.
> It does this by making it so that non-RT VCPUs can't be
> preempted if they are in a critical section, which we
> approximate as having interrupts disabled or non-zero
> preempt_count. Once the VCPU is found to not be in a critical
> section anymore, it will give up the CPU.
> There measures to ensure that preemption isn't delayed too
> many times.
> 
> (I realize that the hooks in the scheduler aren't very
> tasteful, but I couldn't figure out a better way.
> SVM support will be added when sending the patch for
> inclusion.)
> 
> Feedback or alternatives are appreciated.

This is disguisting and completely wrecks the host scheduling. You're
placing guest over host, that's fundamentally wrong.

NAK!

If you want co-ordinated RT scheduling, look at paravirtualized deadline
scheduling.